Over 20 years ago the Federal Food and Drug Administration (FDA) issued a regulation prescribing a test relating to the mechanical strength of spectacle lenses, as defined in terms of resistance to breakage. That test, termed the "Drop Ball Test" by the glass industry, required that such lenses, in a thickness of 2 mm, must survive the impact resulting from a 0.625" (.about.1.6 cm) diameter steel ball being dropped from a height of 50 inches (.about.127 cm). Because annealed lenses are subject to breakage in the FDA test, means were sought to significantly improve the mechanical strengths of the lenses. Two practices have been followed commercially to achieve that need; viz., thermal (air) tempering and chemical strengthening.
Whereas air tempering is a comparatively rapid operation, the process involves relatively high temperatures which may cause distortion of the surface contour of a lens. Furthermore, because glass articles having thicknesses less than about 0.087" (.about.2.2 mm) are very difficult to air temper effectively, the improvement in strength imparted to lenses has not been as great as would be wished. Moreover, in an effort to reduce the weight of glass lenses, there has been the desire to reduce the thicknesses thereof to less than 2 mm. As can be appreciated, such reduction in lens thickness can further decrease the improvement in impact resistance conferred via thermal tempering.
Chemical strengthening, typically contemplating the exchange of large alkali metal ions from an external source, e.g., K.sup.+ ions from a bath of a molten potassium salt, with smaller alkali metal ions, e.g., Li.sup.+ and/or Na.sup.+ ions, in the surface of a glass article at a temperature below the strain point of the glass to form an integral surface compression layer on the article, can more than double the mechanical strength exhibited by a glass in the annealed state. And, because the surface compression layer developed has a depth of about 0.002"-0.003" (.about.0.05-0.08 mm) or somewhat greater, the strengthening effect is operable with glass lenses of small thickness dimensions. Unfortunately, however, the ion exchange reaction proceeds relatively slowly.
Table I presents two white crown glasses currently marketed commercially by Corning Incorporated, Corning, N.Y. Corning Code 8361 has been marketed since the 1940s. Corning Code 8092, having a composition within U.S. Pat. No. 3,790,260 (Boyd et al.), was introduced to the market in the 1970s and was designed to exhibit impact resistances greater than Code 8361 after being subjected to chemical strengthening. The composition of each is expressed on the oxide basis both in terms of weight percent and cation percent.
TABLE I ______________________________________ Code 8361 Code 8092 Weight % Cation % Weight % Cation% ______________________________________ SiO.sub.2 67.65 61.74 62.51 57.14 B.sub.2 O.sub.3 -- -- 0.95 1.50 Al.sub.2 O.sub.3 2.00 2.15 2.79 3.01 Na.sub.2 O 8.00 14.16 8.39 14.87 K.sub.2 O 9.35 10.88 9.34 10.89 MgO -- -- 2.94 4.01 CaO 8.40 8.21 -- -- ZnO 3.50 2.36 11.65 7.86 TiO.sub.2 0.40 0.27 0.73 0.50 As.sub.2 O.sub.3 0.10 0.06 0.10 0.06 Sb.sub.2 O.sub.3 0.45 0.17 0.45 0.06 ______________________________________
Both of those glasses require an immersion in a bath of a molten potassium salt (KNO.sub.3) of about 16 hours to obtain a surface compression layer having a depth of about 0.002"-0.003" (.about.0.05-0.08 mm) with effective mechanical strength A layer of surface compression of that depth has been found to assure that the strengthening effect is not lost when the surface of the glass is subjected to such abuse and abrasion as inevitably occurs in normal service applications.
As was observed in U.S. Pat. No. 3,790,260, supra, the presence of CaO in a glass composition tends to block or otherwise inhibit an ion exchange taking place between K.sup.+ and Na.sup.+ ions. That circumstance results in a shallower depth of the surface compression layer which, while providing a very sizeable initial improvement in mechanical strength, can lead to a substantial diminution of strength as a consequence of surface abuse. Accordingly, it has been deemed most desirable to avoid any significant level of CaO in glasses to be subjected to chemical strengthening.
As can be recognized, the chemical strengthening operation must be carried out after the lens has been ground and polished to a prescribed prescription. The need for a 16 hour immersion in a bath of molten salt has not been well received by the lens dispensers because it adds the equivalent of about a day to the turnaround time required between receipt of a patient's prescription and return of the finished lenses. Hence, there has been a continuing need for glass compositions suitable for ophthalmic lenses which can be chemically strengthened through ion exchange to yield surface compression layers within a period of no more than four hours and, most preferably, no more than two hours, of sufficient depth to survive the FDA Drop Ball Test. Such shortened times would enable a lens dispenser to complete a lens prescription within one day.
Accordingly, the principal objective of the instant invention was to devise glass compositions capable of being chemically strengthened to high values of impact resistance at thicknesses not exceeding about 2 mm and preferably not exceeding about 1.5 mm, coupled with the development of a surface compression layer having a depth sufficient to survive the Drop Ball Test, within a period of time not exceeding about four hours.
Not only has there been a need for a glass lens composition capable of being chemically strengthened in much shorter times, but also for a glass composition containing very low levels of zinc, the preferred compositions being essentially free of zinc. Thus, optical and ophthalmic laboratories have been, and are continuing to be, under increasing pressure to reduce the level of zinc released in the effluent created by their finishing operations. At present, all of the glass white crown lenses and, consequently, the tinted ophthalmic and sunglass compositions based upon them contain substantial concentrations of zinc.
Therefore, a second objective of the subject invention was to not only design glass compositions suitable for optical and ophthalmic applications which can be chemically strengthened within about four hours, but also which will contain very low concentration of ZnO and CaO, the preferred glasses being essentially free of ZnO and CaO.